Biradicaloid mechanisms

In fact, the cycloaddition of butadiene to ethylene, as well as cycloadditions of similar non-polar dienes to non-polar alkenes seem experimentally to be cases where concerted and stepwise (biradical or biradicaloid) mechanisms compete. We have recently discussed a number of cases, such as the dimerization of butadiene, piperylene, and chloroprene, the cycloadditions of butadiene or methylated dienes to halogenated alkenes, and others, where non-stereospecificity and competitive formation of [2 + 2] adducts indicate that mechanisms involving diradical intermediates compete with concerted mechanisms10). Alternatively, one could claim, with Firestone, that these reactions, both [4 + 2] and [2 + 2], involve diradical intermediates1 In our opinion, it is possible to believe that a concerted component can coexist with the diradical one , and that both mechanisms can occur in the very same vessel 1 ). Bartlett s experiments on diene-haloalkene cycloadditions have also been interpreted in this way12). 

Formal insertion of the acetylene molecule into an S—C, Se—C, or Te—C bond occurs when coordinated thio- and selenoaldehydes, selenoketones, and telluroketone (364, M = Cr or W, E = S, Se, or Te, R = H or Ph) react with 1-diethylamino-prop-l-yne (R = Me) or bis(diethylamino)acetylene (R = NEtj). Regiospecihc [2 -I- 2] cycloaddition, for which a concerted biradicaloid mechanism is proposed (227), followed by stereospecihc (Me and Ph cis) electrocyclic ring opening gives the thio-, seleno-, and telluroacrylamide complexes (365) (228-230). 

The concept of biradicals and biradicaloids was often used in attempts to account for the mechanism of photochemical reactions [2,20,129-131]. A biradical (or diradical) may be defined as [132] an even-electron molecule that has one bond less than the number permitted by the standard rules of valence. 

Three of the four photophores, listed in Scheme 3, eliminate nitrogen upon excitation while the biradicaloid triplet state of benzophenone can be reversibly activated, and creates covalent linkage upon excitation-relaxation cycling. This fact and their extremely low reactivity towards protic solvents make them very efficient in the majority of the cases. [7, 19, 20]. Unsaturated ketones can be activated by a similar mechanism, although the secondary processes are much more complex [21]. That mechanism is mainly utilized in steroid hormones possessing unsaturated ketones as intrinsic photophores. 

Using a valence bond scheme parametrized with an effective Hamiltonian technique, it was shown that the mechanistic preference for a synchronous pathway with an aromatic transition state versus a non-synchronous mechanism via biradicaloid intermediate can be controlled by two factors (1) the stability of the long bond in the Dewar valence bond structure, and (2) the softness of the Coulomb interaction between the end methylene groups in the 1,5-diene chain. This means that the mechanism of rearrangement (equation 153) can strongly depend on substituents218. 

Recent DFT calculations by Sarzi-Amade and his collaboratorsmay well have resolved this mechanistic difference between a biradicaloid TS (Scheme 7, path b) and a mechanism involving discrete long-lived free radicals (Scheme 8). Oxygen insertion into the C—H bond of isobutane by DMDO was studied computationally at the unrestricted B3LYP level. Transition structures that were diradicaloid in nature were found to lead to... 

In view of the above exposed facts, to date there is no direct experimental evidence of the intermediary 1,4-dioxy biradical in the decomposition of 1,2-dioxetanes. Therefore, it appears that the asynchronous (biradicaloid or biradical-like) concerted mechanism (merged mechanism) is the one consistent with aU the experimental and theoretical data currently available. 

The n-jr state selectivity observed in the decomposition of 1,2-dioxetanes, even in cases in which the it-it excited state of the carbonyl product possesses lower energy105-108, appears to be best explained by the concerted biradicaloid decomposition of the dioxetanes, or by the intermediacy of an extremely short-lived biradical60. These studies provide strong evidence for the asynchronous, concerted mechanism because a... 

In connection with Eq. (22), yet another important factor differentiates our approach from usual quantum chemical analyses of reaction mechanisms. This difference concerns the fact that while a quantum chemical approach is in principle independent of any external information (all participating species appear automatically as various critical points on the PE hypersurface), in our model that is more closely related to classical chemical ideas some auxiliary information about the structure of the participating molecular species is required. This usually represents no problem with the reactants and the products since their structure is normally known, but certain complications may appear in the case of intermediates. This complication is not, however, too serious since in many cases the structure of the intermediate can be reasonably estimated either from some experimental or theoretical data or on the basis of chemical intuition. Thus, for example, in the case of pericyclic reactions that are of primary concern for us here, the intermediates are generally believed to correspond to biradical or biradicaloid species with the eventual contributions of zwitterionic structures in polar cases. 

Most surprising is the formation of spiro compounds of type 214 when, contrary to electrostatic laws, formally two positively charged carbon atoms of the methylene groups combine. Quantum-mechanical calculations206,211,212 suggest that this process is favoured by the symmetry of molecule frontier orbitals and it may be asynchronous, occurring via the intermediary formation of a biradicaloid species. 

Initially, one of the geometrical isomers, presumably Z, forms faster. The competing thermal E-Z interconversion complicates the situation sufficiently that it is not yet clear whether the transposition reaction forms this geometrical isomer exclusively. If so, the mechanism is likely to be of the dyoiropic kind, involving a shift of both migrating aryl groups and a biradicaloid transition state with a bent or planar bicyclobutane-like structure 24. An alternative that cannot yet be ruled out with confidence is a disilene-to-silylsilylene shift followed by a silylsilylene-to-disilene shift (cf. Sections II.A.3 and II.B.l.b). Both possibilities are displayed in equation 19. 

One of the most important transannular reactions of cyclic alkynes is the Bergman cycliza-lion [67] of enediynes (Scheme 8-15). Nicolaou et al. have shown that the crucial distance between the termini of the enediyne system, allowing spontaneous Bergman cyclization at room temperature, is in the range of 3.1-3.2 A [9cj. Questions about the energy and structure of the biradicaloid intermediate and the mechanism of the biradical formation have stimulated considerable theoretical and experimental work [68]. These investigations and the importance of the Bergman cyclization reaction for the action of a new class of antitumor antibiotics will not be discussed here, as the enediynes are dealt with in considerable detail in Chapter 7. 

A more rigorous definition of a biradicaloid geometry employs the concept of natural orbitals and their occupancies, which are well defined at all levels of quantum mechanical description of molecules in the Born-Oppenheimer approximation at a biradicaloid geometry, two of the ground state natural orbital occupation munbers are approximately equal to unity (the others, of course, are close to two or close to zero). 

As indicated by Table 5.1, the main pyrolysis product under vacuum conditions is TFE. It is formed by the recombination of two [ CF2] units. Buravtsev s observation of a TFE biradicaloid during the decomposition of TFE and the recombination of difluorocarbene indicates that TFE is formed via the mechanism displayed in Scheme 5.3 [41,42]. 

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